This invention relates to an apparatus having double focuses and utilizing a chromatic aberration for accurately sensing the positions observed in the direction perpendicular to a common optical axis of first and second objects separated from each other by a very short distance.
When a single objective lens is used to view the alignment marks of two spaced objects such as a mask and wafer for the purpose of proximity exposure, the gap between the two objects must be made small in order to improve resolving power. In general, if the settable gap is on the order of 10 microns, resolution must be reduced, thus making it impossible to detect the clear image necessary for highly accurate alignment.
Recently, however, detection systems have been proposed in which alignment can be achieved with great precision upon setting a gap of sufficient size between a mask and wafer, e.g., a gap of 10 microns or more. Two examples of such systems can be mentioned. The first is a detecting optical system that relies upon a double focal point lens utilizing the principle of double refraction (see "X-Ray Exposure Devices" in Precision Machines, 1985, vol. 51, No. 12, pp. 34-38). The second is a detecting optical system using rhombic detection (see the specification of Japanese Patent Application Laid-Open No. 61-100930).
Both of these conventional systems make possible highly precise detection by obtaining a clear image, despite a gap size of 10 microns or more. However, since polarized light is employed in the first optical system, the amount of optical information taken in is reduced to a fraction, often a very small fraction. The result is very low resolving power. In the second optical system, the angle between the object plane and the optic axis is not a right angle. Consequently, the area whose image is formed is reduced depending upon the angle and is narrowed to a fraction of the real field of view.
Here a detailed explanation of the relationship between resolving power and gap is in order. The resolving power and focal depth of a single objective lens are defined by the following Rayleigh equations: EQU Resolving power =0.61 .times..lambda./NA (1) EQU Focal depth =.lambda./2n (1-cos .alpha.) (2)
where
.lambda.: wavelength used, PA1 n : refractive index of medium, PA1 .lambda.: angular aperture.
NA: numerical aperture,
It will be appreciated from Eqs. (1) and (2) that resolving power and focal depth are mutually contradictory in the sense that increasing one decreases the other. In other words, if an attempt is made to raise resolving power, focal depth is reduced. For example, letting NA =0.40, .lambda.=436 nm, n =1 (air), we obtain a resolving power of 0.67 .mu.m and a focal depth of 2.6 .mu.m (range). If an attempt is made to observe a mask and wafer simultaneously at this resolving power, the gap required would be less than 2.6 .mu.m. This is a value which is not feasible in view of mask flatness, wafer flatness, the flatness of a process layer formed on the upper surface of the wafer, and an error involved in measuring the gap. The smallest gap presently obtainable in view of the foregoing is about 10 .mu.m.
In order to set a gap of 10 .mu.m, the required focal depth would be 10 .mu.m or more. Accordingly, from Eq. (1), resolving power would be less than 1.1 .mu.m in such case.
Thus, in order to realize highly precise alignment, it is necessary that the alignment marks of the mask and wafer be sensed with greater clarity by raising the resolving power of the objective lens in the detecting optical system. On the other hand, the focal depth must be increased in order to set the gap between the mask and wafer to a usable range (no less than about 10 .mu.m). It is obvious from Eqs. (1) and (2), however, that both of these requirements cannot be satisfied simultaneously.